1. Field of the Invention
This invention relates to wireless communications. Specifically, this invention relates to processing of received signals.
2. Background Information
Pseudorandom noise (PN) sequences are commonly used in direct-sequence spread spectrum (DSSS) communications systems, such as those compliant with the IS-95 over-the-air interface standard and its derivatives such as IS-95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunications Industry Association (TIA) (Arlington, Va.) and used primarily within cellular telecommunications systems. An IS-95-compliant system uses code division multiple access (CDMA) signal modulation techniques to support multiple communications channels simultaneously over the same radio-frequency (RF) bandwidth. When combined with comprehensive power control, supporting multiple channels over the same bandwidth increases the total number of calls and other communications that can be conducted in a system for wireless communications by, for example, increasing the degree of frequency reuse in comparison to other wireless telecommunications technologies.
FIG. 1 provides a highly simplified illustration of a system for cellular telephony that is configured in accordance with practice of the IS-95 standard. During operation, a set of subscriber units 10A–D engage in wireless communications by establishing one or more RF interfaces with one or more base stations 12A–D using CDMA modulated RF signals. Each RF interface between a base station 12 and a subscriber unit 10 includes a forward link signal transmitted by the base station 12 and a reverse link signal transmitted by the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of a mobile telephone switching office (MTSO) 14 and the public switched telephone network (PSTN) 16. The links between base stations 12, MTSO 14 and PSTN 16 are usually carried using wireline connections, although the use of additional RF or microwave links is also known.
Each subscriber unit 10 uses a rake receiver to receive communications from one or more base stations 12. A rake receiver typically includes one or more searchers for locating direct and multipath instances of pilot signals from nearby base stations, and two or more fingers for receiving and combining information signals from those base stations. For example, a description of a rake receiver may be found in U.S. Pat. No. 5,109,390, entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention, and searchers are described in co-pending U.S. patent application Ser. No. 08/316,177, entitled “MULTIPATH SEARCH PROCESSOR FOR SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS”, filed Sep. 30, 1994 and assigned to the assignee of the present invention.
In an IS-95-compliant communications system, the pilot signals are implemented as signals that have PN sequences but carry no data and have constant power over time. A pilot signal that accompanies an information signal may be used by the receiver as a phase reference to support coherent demodulation of phase-shift keying (PSK) modulation schemes such as binary and quadrature PSK (BPSK and QPSK, respectively). Pilot signals are also useful as indications of received signal strength for power control and handoff operations.
In an IS-95-compliant system, a base station differentiates its pilot signal from those of nearby base stations by inserting a unique offset (specifically, an integer multiple of 64 chips) in the generation of its PN sequences. A subscriber unit communicates with a base station by assigning at least one finger to that base station. In order to distinguish the assigned pilot signal, a finger must use the same PN sequence and insert the appropriate offset. It is also possible for base stations to differentiate their pilot signals by using unique PN sequences rather than offsets of the same PN sequence. In this case, a finger would adjust its PN generator to produce the appropriate PN sequence for the base station to which it is assigned.
FIG. 2 shows an architecture for a matched filter searcher suitable for pilot signal searching. Antenna 30 receives a signal that includes pilot signal transmissions from one or more base stations. Receiver 31 downconverts, amplifies, and samples the signal, generating sampled in-phase (I) and quadrature (Q) components of the received signal and delivering the two components to delay chains 36 and 38, respectively. Each delay chain contains N delay elements (labeled DI1–DIN and DQ1–DQN). The output of each delay element is multiplied by a corresponding value of the PN sequences loaded into I and Q tap value chains 35 and 37. The PN sequences are created with I and Q PN generators, and the PN values are loaded or hard coded into the multiplication elements (labeled PNI1–PIN and PNQ1–PNQN) of the tap value chains. Note that in the simple case, the PN values include only +1 and −1, so that inverters (or negaters) may be used in the multiplication elements in place of actual multipliers.
The results of the N multiplications for the I and Q components are delivered to adders 34 and 32, respectively, where they are summed for each component to produce a complex correlation result for that particular alignment of the PN sequence with the received signal (also called a ‘code phase hypothesis’ or simply ‘hypothesis’). The two real components of the complex correlation result are squared and summed in block 33 to produce an energy result which is compared with a predetermined threshold in threshold compare 39. A high-valued energy result indicates a likelihood that the hypothesis is correct, i.e. that a pilot signal was received which has that particular alignment with the portion of the PN sequences contained in the tap elements. As later received samples are shifted into delay chains 36 and 38, an energy result is calculated for each of the corresponding hypotheses.
One alternative to a pilot signal that has constant power over time is a burst pilot signal whose power is gated over time. Examples of systems that have a burst pilot channel structure include those compliant with the IS-856 standard (published by TIA and also known as ‘cdma2000 High Rate Packet Data Air Interface Specification’). In an IS-856-compliant system, for example, the burst pilot signal is time-division multiplexed onto a channel that may also carry control and/or traffic data (i.e. at other times). FIG. 3 illustrates the structure of an IS-856 burst pilot signal, which includes a 96-chip pilot burst in the middle of every half slot (1024 chips). Other examples of discontinuous synchronization mechanisms include the Primary Synchronization Code (PSC) transmitted in the first 256 chips of each slot of the Primary Synchronization Channel in a system compliant with the W-CDMA standard (as described in, e.g., section 5.1 of ITU-R M.1457, ‘Detailed specifications of the radio interfaces of International Mobile Telecommunication-2000 (IMT-2000),’ published by International Telecommunications Union, Geneva, Switzerland) and other W-CDMA time-division multiplexed synchronization mechanisms such as frame timing and burst pilot sequences.
It is desirable to perform acquisition and tracking of time-multiplexed synchronization sequences such as those mentioned above in an efficient manner. For example, it is desirable to realize efficiencies with respect to considerations such as processing time, processing cycles, storage space, flexibility, and programmability. Unfortunately, existing architectures are not suited to perform such operations on time-multiplexed synchronization sequences in an efficient manner.